The present invention is directed to packaging means with an attached tab means held in place by temperature resistant adhesive bonding, in particular a tear-off tab on a portion of the wall of a container. The adhesive is temperature resistant; i.e., the adhesive bond between the tab means and the packaging means is resistant to the effects of sterilization temperatures (130 to 150 .degree. C) as well as low temperatures (-40.degree. C). The present invention is also directed to a process integrated into the production sequence for the adhesive bonding of one or several tabs to a package. Tabs, in particular those for the tearing open of metal containers, such as, for example, beverage cans, cans of different size for food items or the like, are fastened according to the state of the art by means of rivets, for example tubular rivets, to a part of the wall of a hollow body. The disadvantage of this mode of attaching the tabs is that it involves a relatively expensive production method, which is accomplished primarily by a plurality of complex stamping tools. The extreme substrate deformation associated with it frequently requires additional recoating and sealing of the rivet locations. To eliminate the disadvantages relating to the mechanical fastening of tabs, attempts have been made to attach the tabs to the packaging means by adhesive bonding, as described in European patent application 0 065 417 and European patent application 0 078 625. Particularly in European patent application 0 078 625, easily opened lids for containers are described, wherein the tab is bonded to the area to be opened by means of a connecting pivot point in the form of a thermoplastic resin adhesive layer which consists of a copolymer or a polyamide. Exemplary copolymers include copolyesters, copolymers of vinyl ester, ionomers and the like. In order to obtain adequate adhesion at the boundary surfaces at the bonding locations, the metal surfaces must be pretreated and provided with an adhesive primer. The pretreatment of the surface and the provision of adhesive primers are expensive and time consuming. In particular, if the adhesive primer must be dried or possibly burned in, then substrate deformations may result. A further disadvantage arises in the case of adhesion primers which contain solvents which require eternally protected coating equipment and may result in environmental problems. If the primer is applied coil to coil, long and expensive drying tunnels are required to make them economically feasible. An essential disadvantage relates to the backbone polymers of the aforedescribed melt adhesives. Because of this ---- in spite of surface pretreatments and adhesive primers ---- high temperature resistant adhesive bonds cannot be produced. Furthermore, it is not possible to satisfy the aforementioned requirements with the melt adhesives described above.
For example, copolymers of ethylene with vinyl esters, for example vinyl acetate, acrylic acid ester and the like, soften at temperatures between 60.degree. and 90.degree. C. and, due to the relatively low softening point, begin to creep.
Polyamides based on fatty acids, for example dilinolic acid and hexamethylene, have in the case of low or intermediate molecular weights, in spite of their good wetting properties--due to their relatively low crystal melting points--only a limited heat resistance, which affects the adhesive bond produced while in storage. In the case of the higher molecular weights, due to the higher melting points, provide higher strengths at elevated temperatures but are very difficult to process, as application temperatures of at least about +240.degree. C. are required and they may be used only with a limited number of substrates.
Copolyamides of monomers, such as for example, caprolactam AH salt, adipinic acid esters of the hexamethylenediamine of 11-aminoundecanic acid, lauric lactam, exhibit high melt viscosity values and are therefore used in the powder form only. However, they have inadequate flexibility.
Copolyesters, such as for example tere- and isophthalic acid esters of glycols. for example 1,6-hexanediol, (a) yield in the case of low viscosities (n.ltoreq.0.4) adhesive layers which adhere poorly and break easily, (b) require in the case of medium viscosities (0.4.ltoreq.n.ltoreq.0.85) very high processing temperatures (+250.degree. C. or higher) and their adhesive layers are not resistant to hydrolysis, and (c) they melt with difficulty at high viscosities (n=0.9 or higher) and wet the adhesive surface poorly.
Furthermore, the heat sensitivity of these backbone polymers, particularly in the case of polyamides, is a critical factor and their so-called "heat history" is very difficult to handle. The adhesive bonds so prepared between identical and different substrates deteriorate significantly, for example, during sterilization at 130.degree. C./60 min.
Packaging means are defined in the context of the present invention as containers, cartons, boxes, cans and the like, used for the packaging of food and condiments, pharmaceuticals, cosmetics, technical products, beverages, mineral oils, solvents, and water. A particular area relates to the hollow bodies encountered in the marketplace, in particular, cylindrical cans and containers for liquids, such as beverages, oils, but also of goods to be maintained fresh, such as, for example, meat and sausage wares, prepared meals, nuts and the like, preferably of tin plate and aluminum.
The packaging means may be made of different substrates and substrate combinations. These include metals, for example steel and aluminum sheets and foils, which may be laminated with plastic and/or cellulose substrates and/or coated. Plastics, in particular, thermoplastics, such as polyethylene, polypropylene, polyvinylchloride, polyvinyledene chloride, ethylene-vinylester copolymers, polyvinyl alcohols, are also suitable and may be used by themselves or as composite substrates with metal and/or cellulose substrates. Finally, packaging means comprised of cellulose materials, such as paper, cardboard or the like, are appropriate. They may be employed alone or in combination with metal and/or plastic substrates, or provided with a lacquer and/or wax layer.
Tabs as defined herein include functional parts or handles for packaging means, fixedly attached to the packaging containers and serving to render the opening of the package, in particular containers, easier, as well as providing transportation and/or suspension functions. It is assumed that the packaging means or the locations, for example, lids, to which the tabs are attached to the packaging means, are configured, for example, by reductions in cross section (notching) in a manner such that the packaging means may be torn open with the tab. As an example of such containers, Swiss Pat. No. 417 376 is cited. The tabs may be made of the same substrate as the packaging means. They are often reinforced by the inclusion and/or embedding of reinforcing materials, such as fibers, so as not to be damaged or destroyed by one of the above-mentioned functions.
In order to be able to perform one of the above functions, in particular the tearing open function, it is necessary to render the bond between the packaging means and the tab strong enough so as not to be weakened or destroyed by the application of force. This requirement could be satisfied by a riveted joint only. The adhesive bond described in European patent application No. 0 078 625 between the packaging means and the tab established with standard backbone polymers performs this function in part only if the adhesive surface is pretreated and provided with an adhesive primer. In case of increased temperatures, such as in the case of prepared meals, the force required for the tearing intended to act through the tab on the predetermined fracture location, leads to fracture or shearing off at the adhesive bonding surface. Similar processes occur during the application of force at low temperatures, during transport and/or upon the hanging of a packaging means.
The heat resistant adhesive bond between the packaging means and the tab must have an adhesive strength such that during the application of force in the tensile, tensile-shear and/or peeling mode at different temperatures the adhesive strength must be greater than the force required for the breaking or tearing of the packaging means. Only then is a tab an acceptable operating part of a packaging means, in particular a container.